1. Technical Field of the Invention
The present invention relates to an apparatus and method for determining patterns of damage being caused in a rolling contact element such as rolling bearings including a roller baring, and in particular, to a design technique for clarifying the relationship between a damage pattern in a contact area of the rolling contact element and stress to avoid the rolling contact element from abnormal damage.
2. Description of the Related Art
A rolling element, playing a role as a mechanical element, that is, among others, a roller bearing in common use suffers from damages in various patterns. Despite the bearing playing a role as an important component part, there are many probabilities in which no definite solution is obtained to clarify what is to be made to avoid such damages. What can be solely considered in design on a preliminary stage includes only two aspects of obtaining a rolling fatigue life upon calculation of a fundamental dynamic rated load (generally designated by “C”) and a way of precluding the occurrence of permanent deformation by calculating the fundamental dynamic rated load. That is, with a consequence of the fatigue life, a final damage pattern results in pitching or flaking. Moreover, when experienced with permanent deformation, the damage pattern results in brinelling (in a brinelling indentation). Despite such brinelling, the bearing suffers from, in addition to such a damage pattern, other damages in various patterns. For instance, these include a pseudo indentation (false brinelling), cracks, chips, fretting, incisura and galling or the like. All of the causes for these phenomena are clarified but no distinct solution is provided for which of a threshold value of the cause results in the occurrence of damage to the bearing. Therefore at the current status quo, it is quite difficult to make a design of a bearing in a preliminary stage to avoid such phenomenon.
In addition to this status quo, recently, roller bearings of various auxiliary-unit component parts such as an alternator of an automotive engine, an air conditioning unit and idler pulley have come to be used in recent years under severe conditions involving vibrations and temperatures or the like. This results in exposure of flaking accompanied by variation in tissue under new patterns. Flaking takes place in any area of the component parts such as an outer race, an inner race and balls (or rollers) and has a feature differing from a fatigue life experienced by a commonly used roller bearing. This is a damage pattern wherein once the fatigue life occurs, flaking occurs on the component part of the bearing within a shortest time period (in the order of approximately 1/100 to 1/1000 times that of the related art). A feature of this damage pattern is that is does not exhibit a tissue (in the form of a so-called DEA: Dark Etching Area) which is seen to be upon subject to an etching process with nital liquid as done in a fatigue life test of the related art but to exhibit another tissue (in a white band that is a so-called WEA: White Etching Area).
In a field of bearings, this flaking is called brittle flaking or white-banded flaking with a view to differentiating the same from the fatigue life referred to in the related art. FIG. 1 shows an example of a raceway track of a bearing entered with flaking in a white-band. To be different from a consequence in which all of roller bearings, subjected to rolling life tests conducted on flaking based on a fatigue in the related art practice, are caused to suffer from fatigue breakdown on a final stage, no mechanism of such brittle flaking has been determined yet. Such brittle flaking has a specificity wherein flaking occurs on the bearing within an extremely short time period when experienced breakdown under certain circumstances of recurrence test conditions but no brittle flaking takes place under conditions with no breakdown. Therefore, under a status where a first aid measure is taken with no distinctive scientific basis, no full-scale measure has been taken in this status quo.
A major candidate on a mechanism of such white-banded flaking (brittle flaking) is based on a hydrogen theory. That is, this theory stands on the ground that a ball is caused to slip due to stress such as vibrations applied to the bearing in use and heat and pressure develop in the bearing to cause the decomposition of grease into hydrogen with the resultant occurrence of flaking due to hydrogen brittleness. On the ground of such a theory, various attempts have heretofore been taken to avoid the occurrence of flaking by applying a raceway with an oxide film so as to preclude the separation of hydrogen from grease as disclosed in Japanese Patent Publication No. 6-89783 or to prevent generated hydrogen from entering the raceway. However, as a result of various tests conducted by the present inventors, these attempts have not always been successful in the prevention of flaking. Upon recurrence tests under other conditions than those of the tests conducted on the above attempt with a desired effect, instead, no effect is found and a worse phenomenon is turned out. Certainly, a bearing made of steel forcedly added with hydrogen in a preceding step undergoes white-banded flaking within a short time interval during a test in most of the conditions but no conclusion was obtained in the bearing wherein grease is decomposed into hydrogen during normal operation to cause hydrogen to penetrate into steel resulting in white-banded flaking due to hydrogen brittleness.
Further, another mechanism, a stress theory (a vibration theory in the sense of stress) has been advanced. That is, this theory is a way of thinking to attempt for explaining the occurrence of flaking in terms of stress. This theory falls in the same contradiction as that of the stress theory in that no distinction is possible between flaking based on the stress theory and a commonly experienced fatigue life (accompanied by DEA) based on shear stress. Further, upon various tests conducted on bearings with a real machine (automobile) by the present inventors, flaking has occurred in the bearings and research work has been conducted to find out the relationship between stress and flaking. As a result of such research, the present inventors have determined a fact that during operation of the alternator in a low belt tension, the belt tension is zeroed (to be less than 0 Kg) due to adverse affect of an inertial force caused by engine deceleration and, accordingly, white-banded flaking has appeared only when a drop clearly occurred in load exerted to the bearing at a value of 0 Kg. Such an exemplary case is hard to be explained in terms of the stress theory. Although the other explanation is omitted herein, the stress theory falls in the same contradiction as the hydrogen theory and, therefore, even if countermeasure had been taken on bearings on the ground of the stress theory, white-banded flaking had still occurred in the bearing on a real machine.
As set forth above, none of the mechanisms meets actual conditions for brittle flaking to take place in new types of damage patterns and it is completely unclear to determine which of stress factors of a real machine adversely affects on the occurrence of flaking. Accordingly, a situation stands on the ground with no capability of taking measure on a design of the bearing. In addition, a modern engine adopts a serpentine drive system with a plurality of pulleys driven by a single belt for the purpose of minimizing an engine with lightweight. With such a structure, an issue progressively arises in an increase in belt tension, belt resonance and promoted engine vibration or the like and under such conditions bearings suffer from stress in a complicated pattern. None of the theories, proposed in the past as mentioned above, have made solutions to the occurrence of such brittle flaking. In spite of the roller bearing playing a role as the important mechanical element component part, not only full-fledged measure had not be taken to address the issue of brittle flaking but also even the mechanism for addressing such issue had not been established.
Further, certain damage with mild brinelling has come to be found in a bearing installed on an alternator employing a modern serpentine drive system (see FIG. 2). While this damage looks like false brinelling (also referred to as mild fretting) at the first glance, this damage is not actually associated with an incidence of wear and formalized as brinelling. Among bearings subjected to mild brinelling, some bearings have come out to consequences wherein due to adversely affect resulting from such mild brinelling, balls of the bearings tend to be worn each in a band-like configuration to evolve into a pattern liable to be mistaken to be grinding burn. So to say, as mild brinelling progresses (as a primary failure), the bearing has encountered with damage in a new mode such as band-like wear and pseudo grinding burn or the like as a secondary failure (FIG. 2 shows a photograph of one of a large number of mild brinelling occurring on a raceway track of an inner race of a ball bearing whose ball has encountered with band-like wear).
In the related art practice, although the resulting indentation has been explained as false brinelling resulting from vibration exerted to the ball during a stop of rotation of the alternator, no probability occurs for vibration to be imparted to the bearing because the alternator comes to a halt during a stop of the engine. Although there is a common idea in that the bearing is subjected to vibration accompanied by an indentation during a transportation of an automobile on a ship or a trailer, no way exists for mild wear to take place because damage of this kind has not been recognized in bearings of the related art belt drive system prior to the employment of a serpentine drive system and even during a halt of an engine, belt tension has prevented the bearings from being subjected to undulation resulting from vibrations caused during transportation. Thus, no chance takes place for mild wear to occur (with damage accompanied by a wear phenomenon being defined in the related art to be false brinelling or fretting). Moreover, upon observing the picture of FIG. 2 in detail, although a surface seems to be indented, an indentation has a bottom on which a grinding mark is left and although plastic deformation is present, no wear is present.
Accordingly, the present inventors had made it clear that this mark is a shallow indentation in the form of mild brinelling so to say. If the primary failure (in the form of mild brinelling) can be suppressed, then, no various secondary failures induced from such a primary failure take place and, therefore, a need arises to suppress the occurrence of mild brinelling but even such a mechanism has not been established in the related art.
As set forth above, although the roller bearings take the various damage patterns, what is a guideline to be useful for designing the bearing in advance includes only flaking in a fatigue life and brinelling in permanent deformation and other patterns have had no guideline to be useful for designing the bearing. Further, in spite of an extremely short operating life resulting from brittle flaking caused in a modern bearing as compared to that of a commonly experienced fatigue life (no problems had arisen in alternators in actual practice), no mechanism for explaining such a phenomenon has been established and a situation stands with no capability of taking appropriate measure to address the issue. With no alternatives, woefully inefficient methods in irregular measures have heretofore been taken in this status quo for each of the auxiliary units of the engines upon conducting a test of the bearing on a real machine for confirmation. This results in wasteful efforts involving a step of manufacturing a bearing with an unnecessary increase in size or a bearing with increased prevision. Even with such attempts, such an inconvenience could not be completely addressed. Also, no mechanism for the occurrence of mild brinelling in a modern bearing had been clarified in the status quo. However, the damage patterns include damages in other patterns, resulting from other causes than the mechanical factors (e.g., seizing and electric corrosion), which are scientifically clarified. Thus, description of such damage patterns is herein omitted.
In such a way, although the roller bearings are involved in a variety of mechanical damage patterns, none of the relationships (except for a part thereof being clarified) between a source of cause and stress has been clarified. Attempts have been taken on measures relying on know-how and no measure had addressed all of the issues in advance when designing the bearing.